1. Field of the Invention
The present invention relates to a catalyst deterioration-determining device and method for determining deterioration of a catalyst that purifies exhaust gases from an internal combustion engine, and an engine control unit.
2. Description of the Related Art
Conventionally, there has been proposed a deterioration determination device of this kind in Japanese Laid-Open Patent Publication (Kokai) No. H05-133264. In this deterioration determination device, an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor are disposed in an exhaust passage on respective upstream and downstream sides of a three-way catalyst, for detecting the air-fuel ratio of a mixture burned in the engine, and the deterioration of the catalyst is determined in the following manner: First, the air-fuel ratio of the mixture is controlled to a leaner value than the stoichiometric air-fuel ratio, whereby oxygen in exhaust gases is caused to be fully adsorbed in the three-way catalyst. Then, the air-fuel ratio of the mixture is switched to a richer value than the stoichiometric air-fuel ratio, whereby unburned fuel in the exhaust gases is supplied to the three-way catalyst as reducing agents, to cause the same to react with oxygen adsorbed in the three-way catalyst. Thereafter, when the downstream air-fuel ratio detected by the downstream air-fuel ratio sensor becomes substantially equal to the upstream air-fuel ratio detected by the upstream air-fuel ratio sensor, the difference between the stoichiometric air-fuel ratio and the upstream air-fuel ratio at the time is multiplied by a time period (first time period) which has elapsed from the time of switching the air-fuel ratio of the mixture to the richer value, and a detected intake air amount, whereby a first oxygen amount indicative of the amount of oxygen adsorbed in the three-way catalyst is calculated.
Next, the air-fuel ratio of the mixture is switched to the leaner value again, whereby oxygen in exhaust gases is caused to be adsorbed in the three-way catalyst. Thereafter, when the downstream air-fuel ratio becomes substantially equal to the upstream air-fuel ratio, the difference between the upstream air-fuel ratio at the time and the stoichiometric air-fuel ratio is multiplied by a time period (second time period) which has elapsed from the time of switching the air-fuel ratio of the mixture to the leaner value, and a detected intake air amount, whereby a second oxygen amount indicative of the amount of oxygen adsorbed in the three-way catalyst is calculated. Then, as the average value of the first and second oxygen amounts is smaller, it is determined that the deterioration of the three-way catalyst is in a more advanced state.
As described above, the conventional deterioration determination device determines deterioration of the three-way catalyst, based on the first and second oxygen amounts calculated according to the first and second time periods which are taken before the downstream air-fuel ratio and the upstream air-fuel ratio become equal to each other after the air-fuel ratio of the mixture is switched between the leaner value and the richer value. However, if the output characteristics of the upstream and downstream air-fuel ratio sensors, particularly, the gains thereof, are different from each other, timing in which the downstream air-fuel ratio becomes actually equal to the upstream air-fuel ratio cannot be accurately grasped, which makes it impossible to accurately measure the aforementioned first and second time periods, and hence makes it possible to carry out accurate deterioration determination. Further, since the two air-fuel ratio sensors are disposed with the three-way catalyst therebetween, and exhaust gases having reacted flow into the downstream air-fuel ratio sensor and hence the composition and the activity of exhaust gases are different between the two air-fuel ratio sensors. This sometimes makes the output values from the two sensors exposed to exhaust gases having the same air-fuel ratio different from each other even though the output characteristics per se of the sensors are not different from each other. In this case as well, it is impossible to accurately grasp timing in which the downstream air-fuel ratio becomes equal to the upstream air-fuel ratio, which degrades the accuracy of the deterioration determination.